Background: The brain and gut communicate bidirectionally via immune, neurological, and endocrine pathways, which is termed the "brain-gut interaction."Recent studies of gut microbiota as a mediator of this interaction have provided a growing body of scientific evidence that suggests that the gut microbiota influences stress and emotional responses and stress-related disorders. Summary: Major advances in analytical methods have led to an increased number of studies that combine gut microbiota and neuroimaging, mainly magnetic resonance imaging, to elucidate the mechanisms. Observational studies have been done to examine brain characteristics related to gut microbiota profiles, and intervention studies have examined brain changes related to probiotic intake. Studies of healthy subjects using negative emotional stimuli have shown that the pattern of emotional response differs depending on the gut microbiota profile and that probiotic intervention can modulate emotional response and be a buffer against the negative effects of stress. In studies on irritable bowel syndrome (IBS), a typical psychosomatic disorder, IBS-specific gut microbiota were reported to contribute to visceral irritability and pain by affecting the subcortical regions. Studies on psychiatric disorders revealed that a relative abundance of Bacteroides that produce γ-aminobutyric acid in feces was associated with a change in brain function specific to depression and that gut microbiota have an influence on abnormalities in the reward system of attention-deficit/hyperactivity disorder.
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